The present invention relates to the field of electrical storage batteries, and, in particular, relates to a lead-acid electrical storage battery which is provided with an anti explosion protective device.
Lead-acid batteries, whose electrodes are made of lead, and which are filled with an electrolytic fluid comprising sulphuric acid, are well known, and are often used in automotive applications or the like. Such batteries, when they are being charged, evolve hydrogen gas within their battery casings in fairly large quantities. Therefore, such lead-acid electrical storage batteries usually are provided with a gas venting exhaust passage, which connects the interior of the battery casing to the atmosphere for venting the gases generated within the battery casing to the atmosphere.
Thereby a danger has arisen, in that if a naked flame should be exposed near a lead-acid electrical storage battery when it is being charged, or if an electrical spark should occur near to the battery, due, for example, to a high voltage electrical potential generated by a high voltage device such as a coil, or alternatively due to static electricity generated by friction or the like, it may well occur that the hydrogen gas that is being emitted from the gas venting exhaust passage of the storage battery should become ignited, and the flame front traveling through this hydrogen gas may well enter inside the battery casing, moving back up the exhaust outlet passage formed in the battery casing, and may ignite a fairly substantial quantity of explosive hydrogen gas-oxygen mixture filling the battery casing, thus causing a disastrous explosion. Such an explosion, apart from destroying the storage battery, will scatter the corrosive sulphuric acid contained in the electrolytic fluid around the vicinity of the battery.
Therefore, in consideration of the above described difficulty with conventional lead-acid storage batteries, there has already been proposed, and practiced, an improvement to a lead-acid storage battery, in which, across the gas venting exhaust passage of the battery casing, there is provided a flame intercepting filter, made of a sponge-like coarse material. This flame intercepting filter in the gas venting exhaust passage acts as an effective bar to the travel of a flame front through the hydrogen gas which is being evolved and vented through the passage, and thus prevents this flame front from entering into the interior of the battery casing. Thereby, it is effectively prevented that a flame front starting in the evolved hydrogen gas which is on the atmosphere side of the flame intercepting filter should travel back to the interior of the battery casing and cause an explosion therein.
This solution to the above described problem has been effective and has been widely practiced. However it does not completely solve the explosion problem of lead-acid electrical storage batteries, because it sometimes happens that, during the charging process of such a lead-acid electrical storage battery, a mist composed of small droplets of electrolytic fluid suspended in the hydrogen gas which is being passed through the gas emitting discharge passage is formed, especially if the charge rate of the battery is high, and droplets of electrolytic fluid deposited from this mist may form an electrically conducting layer on the wall of the gas venting exhaust passage, on both sides of the flame intercepting filter. This conducting layer will retain its conductivity, due to the affinity of sulphuric acid for atmospheric water vapor, i.e., its hygroscopicity. Thus, if a source of high voltage electricity is brought close to the exterior of the electrical storage battery, a possibility arises of sparking electric discharge between this high voltage source and the abovementioned electrical conducting electrolytic fluid layer.
If this sparking electrical discharge occurs on the atmospheric side of the flame intercepting filter in the gas venting exhaust passage, then no problem will occur, because any flame front, as explained above, passing through the hydrogen gas which is being evolved, outside the battery, will be prevented from entering within the battery casing, by the flame intercepting filter. Thus, any explosive mixture of hydrogen gas and oxygen within the battery casing will not be ignited, and no explosion can occur.
On the other hand, however, if the conducting layer of electrolytic fluid deposited on the wall of the gas venting exhaust passage is discontinuous at a portion within the battery casing interior, i.e., to the battery casing interior side of the flame intercepting filter within the gas venting exhaust passage, an electric spark may occur at the discontinuous gap portion from a high voltage electrical source, and, accordingly, an explosive mixture of hydrogen gas and oxygen filling the battery casing may be ignited. Thereby, a disastrous explosion may occur, even though the abovementioned flame intercepting filter is located in position within the gas venting exhaust passage.